Filament Spool Remaining Calculator
A partly used filament spool gives no easy way to read how much is left, but a kitchen scale solves it. Weigh the whole spool, subtract the empty spool weight, and the remainder is your filament in grams. This calculator then converts that to remaining length in metres using the grams per metre for your diameter and material density, so you can tell at a glance whether a print will finish before the spool runs out.
Filament remaining formula
remaining grams = total weight - empty spool weight
radius (cm) = diameter (mm) / 2 / 10
grams per metre = pi * radius^2 * 100 * density
remaining length (m) = remaining grams / grams per metre
Subtracting the spool gives filament mass. Grams per metre comes from the filament cross-section times 100 cm times density, and dividing remaining grams by it gives length.
Worked example
A spool weighs 650 g with a 220 g empty weight, so 430 g of filament remains. For 1.75 mm PLA at 1.24 g per cubic cm, grams per metre = pi * 0.0875^2 * 100 * 1.24 = 2.98 g. Remaining length = 430 / 2.98 = 144.18 m. Plenty for most small prints.
Filament spool remaining: frequently asked questions
How do I find how much filament is left on a spool?
Weigh the spool on a kitchen scale, subtract the empty spool weight (printed on the label or measured from a finished spool), and you have the remaining filament weight in grams. To convert that to length, divide by the weight per metre, which depends on diameter and material density.
What is the empty spool weight?
It is the mass of the bare reel with no filament, often listed on the manufacturer's spool or label and commonly around 180 to 250 g for a 1 kg plastic spool. Because it varies by brand, this calculator takes the empty spool weight as a user-editable input. Save the weight of an emptied spool of each brand you use.
How is remaining length calculated?
Remaining length equals remaining filament grams divided by the filament's grams per metre. Grams per metre comes from the cross-section area times density: for 1.75 mm filament the area is fixed, so the figure depends mainly on material density. The calculator computes grams per metre from your diameter and density inputs.
Is weighing accurate enough?
Yes, for planning. A scale accurate to 1 g gives remaining filament to within a metre or two, which is plenty to know whether a print will finish. The main uncertainties are the empty spool weight and the exact material density, both of which you control as inputs here.
Sources and method
- Remaining mass is a simple subtraction; length comes from the cylinder volume and mass-equals-density-times-volume identities. These are standard geometry and physics.
- National Institute of Standards and Technology: Office of Weights and Measures (SI units, density). Enter spool weight and density from your own measurements and data sheet.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.